Which has always impressed me and I use the feature.
Increased conservatism?
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DAN flight guidelines are super conservative. If you understand deco you can make an informed choice based on your residual nitrogen vs the pressure differential gradients. Not to mention anyone with decent altitude training understands how to plan and account for increased altitudes before during and after a dive. As an open water / altitude diver I routinely had to calculate how long of a surface interval I needed before heading over the mountains and back home. Adding 100% o2 for conservatism while doing so is a good idea, but more or less going to 8k should be fine as long as you are D or higher on navy tables.
A time I backed off my limits was when I had poor gas planning on a shallow shore dive off florida Gulf Coast. I was diving sidemount hunting fossils when I ran critically low on air (8 ft3 in both tanks) and had to surface and do a 1000 yard surface swim against tide in a sidemount rig. I've become more conservative on my turn pressures following 1/3s even in recreational settings since. I used to show up on the boat at as close to 500psi as possible, now I just bring slightly more gas and show up with a good 40ft3 left.
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Hello Blac,
There are a couple of points in your post that I would like to pick up on.
I guess this hinges on your definition of super-conservative, but I am not sure I would characterise the guidelines in this way. They are, in fact, based on some fairly extensive test programs involving hundreds of experimental dives followed by depressurisation to altitude at various intervals after the dive exposures. The derived limits may be an hour or two longer than the pre-flight interval at which the associated incidence of DCS was found to fall to zero or close to it with reasonable confidence, but I would not call that "super" conservative. Moreover, on some dive profiles at least, when you wait for progressively shorter intervals before flying the DAN data suggests that the risk starts to rise quite steeply.
I am not trying to be provocative, but I would be sceptical about this. I have a moderate understanding of deco, but prefer to stick to recommendations based on measurements of risk in large populations of divers rather than try to tailor individual limits to myself.
Of course, I am happy to acknowledge the fact that you have applied whatever formula it is that you use to your own activities, and that it has appeared to work. But the value of big studies involving large numbers of subjects is that we can be more confident that the results are generalizable.
Simon M
There are a couple of points in your post that I would like to pick up on.
I guess this hinges on your definition of super-conservative, but I am not sure I would characterise the guidelines in this way. They are, in fact, based on some fairly extensive test programs involving hundreds of experimental dives followed by depressurisation to altitude at various intervals after the dive exposures. The derived limits may be an hour or two longer than the pre-flight interval at which the associated incidence of DCS was found to fall to zero or close to it with reasonable confidence, but I would not call that "super" conservative. Moreover, on some dive profiles at least, when you wait for progressively shorter intervals before flying the DAN data suggests that the risk starts to rise quite steeply.
I am not trying to be provocative, but I would be sceptical about this. I have a moderate understanding of deco, but prefer to stick to recommendations based on measurements of risk in large populations of divers rather than try to tailor individual limits to myself.
Of course, I am happy to acknowledge the fact that you have applied whatever formula it is that you use to your own activities, and that it has appeared to work. But the value of big studies involving large numbers of subjects is that we can be more confident that the results are generalizable.
Simon M
Simon I appreciate the cautious approach that you and several other divers successfully use. I also understand the skepticism related to flying after diving, but I have applied a great deal of cautious study before coming to my conclusions.
Using altitude procedures and decompression modelling software I can track and plan for the difference in pressures in my body tissues. An airliner cabin is pressurized to a minimum of 11psi, or the equivalent of 8000 feet. Using my flight times and altitude tables I can create a simulated deco profile within MultiDeco of my flight. I make sure that I don't cross a gf line of 80/80 before I fly, taking into account the rapid off gassing during ascent to cruising altitude.
DAN guidelines are conservative to the point that no one would have to worry about any of this, their dcs risk is near-as-makes-no-difference zero. That's why I called them super conservative though I can see why you would debate that description.
I don't recommend a diver flaunt flight guidelines without thorough understanding and planning. The existing guidelines are extremely safe, but like all incidences of increased conservatism it comes at the cost of decreased efficiency. If one is willing to do the research and planning they will find they can travel safely much soober than Dan guidelines in most cases.
I thought the way to work these things out was to do take some numbers from tables based on special gases, have a look for a bit of a pattern and then add a bit depending on how you are feeling after a dive. That's doing it right.
Depending on route requirements and aircraft type, the cabin altitude may go up to 10 000 feet on a routine flight.
Partial and total pressure loss are far more common than one might think. Planning a perfectly safe situation and then getting bent because a 20$ valve develops a leak is not how I choose to live, YMM as always V.
blac86:
Hello again Blac,
This approach presupposes that tissue gas loading is the pivotal issue in determining pre-flight surface interval, and that you have a validated ascent rule for this scenario which I'm pretty sure no one has. But it fails to account for another potentially important issue in flying after diving which is the presence of bubbles in blood or tissues after the dive(s). These will grow in accordance with Boyle's Law as ambient pressure decreases, and as the surrounding tissue becomes supersaturated during ascent to altitude they will also grow in response to inward diffusion of inert gas. So, it is probably not simply a matter of "inert gas accounting". There is, of course, no way we can account for pre-existing bubbles on an individual basis, which brings me back to the importance of big studies that define population risk for such guidelines. I do not deny for a moment that your method appears to work for you, but you need to be very cautious in generalising it. There is potentially marked variability in tendency to bubble formation among divers.
blac86:
Again, I would draw your attention to the fact that risk rises steeply when you start undercutting the recommendations by "much". It depends on the definition of "much". See the diagram below. These are DCS incidence data from the man trials conducted by DAN for altitude exposure after repetitive diving (where the recommended preflight surface interval is 18 hours). You can see that once you get below 16 hours the risk really does start to rise quickly.
Simon M
I prefer to stick to the published guidelines from DAN with regards to flying.
Why? I might be able to fly safely sooner but how do I know it is actually safe? In the absence of personal Doppler testing prior to and during flights to confirm the lack of bubble formation, I will not ever know how close I am cutting it until I suffer DCS symptoms in a situation where I might find I can't do anything to mitigate them. Better to err on the safe side instead IMHO.
Why? I might be able to fly safely sooner but how do I know it is actually safe? In the absence of personal Doppler testing prior to and during flights to confirm the lack of bubble formation, I will not ever know how close I am cutting it until I suffer DCS symptoms in a situation where I might find I can't do anything to mitigate them. Better to err on the safe side instead IMHO.
Not that I ever expect to push the limits of the DAN guidelines, but if you ever felt the onset of DCS on a plane, I wonder how hard it would be to get O2 from a flight attendant?
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